This invention relates generally to data recovery systems and more particularly to systems utilizing partial response maximum-likelihood (PRML) detection.
Data recovery systems are used in a variety of applications. In one application, such system is used to recover (i.e., read) data stored on a magnetic disk of the type used as hard drives in computer data storage systems. In one type of such data storage system, a predetermined preamble pattern is written onto the magnetic disk before the start of any block of data to be stored. The preamble pattern is used during the read operation to acquire initial gain, frequency and phase lock to the data read from the disk. Thus, when data is read from the disk, the preamble is used by the read channel during a preamble recovery mode to obtain clock (i.e., sampling or timing) pulses for a subsequent data recovery mode.
The data read from the magnetic disk is typically passed through an analog equalization filter prior to passing to succeeding stages of the data recovery system. The analog equalization filter is configured to shape the sequence of preamble pulses and data pulses into pulses having a predetermined waveform characteristic. The analog equalization filter has non-linear change in phase as a function of frequency characteristic. The analog equalization filter produces pulses that are fed to an analog-to-digital converter . The analog to digital converter converts samples of the shaped pulses into corresponding digital words in response to the clock pulses (i.e., sampling pulses) fed to a clock input of the converter. In one arrangement, such as in U.S. Pat. No. 5,220,466, a pair of feedback loops is coupled between an output of the analog to digital converter and the clock pulse input to such converter. A first one of the feedback loops is used during the data recovery mode and includes a digital filter fed by the digital filter words produced by the analog to digital converter. The digital filter has coefficients selected to compensate for the non-linear change in the magnitude and phase as a function of frequency characteristic of the equalization filter. A tracking gain and phase control section is fed by the digital filter and is responsive to the digital words representative of the data pulses for producing the clock pulses for the analog to digital converter during the data recovery mode.
However, in order to rapidly synchronize the clock pulse frequency and phase to the preamble pulse frequency f.sub.p and phase during the preamble acquisition mode, the digital filter is by-passed (i.e., the first feedback loop is disabled) and a second one of the pair of feedback loops is used. That is, the second feedback loop is used during the preamble acquisition phase to lock onto the frequency and phase of the preamble for subsequent switch-over to the first feedback loop. The second feedback loop includes an acquisition gain and timing control section responsive to the digital words representative of the preamble pulses for producing clock pulses for the converter which are synchronized (i.e., locked) in frequency and phase to the preamble pulses.
The use of an asymmetric digital filter can improve data recovery performance. That is, because the analog equalization filter may have a non-linear phase vs. frequency characteristic, an asymmetric digital filter may provide better compensation. However, when switching from the preamble acquisition mode to the data recovery mode, an undesirable a phase step change may occur in the system.